Vehicular air conditioning systems are almost uniformly based on the compression cycle. In a compression system, refrigeration is produced by the repeated process of compression, condensation, expansion and evaporation of a refrigerant, which is a liquid with a relatively high vapor pressure. In order for the compression system to operate, a substantial amount of energy must be supplied to power the compressor, which serves to compress the refrigerant vapor. The energy required to power the compressor can represent up to fifteen percent of an automobile's fuel consumption. Furthermore, the refrigeration effect is reduced when the engine driving the compressor is operating at low revolutions, and no refrigeration is produced at all when the engine is not operating.
An additional drawback to conventional compressor-based air conditioning systems is their widespread utilization of chlorofluorocarbons ("Freon") as the refrigerant. The use of Freon is currently disfavored since it has been shown to be an ozone-destroying agent, and leaking Freon from malfunctioning or junked automotive air conditioners is believed to have a substantial adverse effect on the stratospheric ozone layer. Other less environmentally harmful refrigerant compounds may be substituted for Freon in compression systems, but at a loss of performance and an increase in cost.
The use of the absorption cycle to effect refrigeration has been known for well over one hundred years. The principle of operation of the absorption system is similar to that of the compression system; however, instead of a compressor, the absorption system utilizes a generator filled with a mixture of at least two intermiscible substances (a refrigerant and an absorbent) and an absorber, in which the absorption of the refrigerant by the absorbent occurs. It is necessary to the operation of the absorption system that the refrigerant have a much higher vapor pressure than the absorbent so that when heat is supplied to the generator, the more volatile refrigerant evaporates and flows to the condenser.
Refrigeration in an absorption system is produced by sequential condensation, expansion and evaporation in a manner closely similar to the compression cycle. The principal advantage of the absorption system over the compression system is that due to the substitution of a liquid pump for the compressor, only a small amount of mechanical work is required for operation. The absorption system does, however, require a source of heat to produce evaporation of the refrigerant within the generator. Consequently, absorption systems are well suited to applications where it is inconvenient or inefficient to supply mechanical or electrical power to the refrigeration system, but where a source of heat is readily available.
The prior art contains descriptions of a variety of vehicular refrigeration/air conditioning systems based on the absorption cycle in which the waste heat contained in the engine exhaust gases is utilized to cause evaporation of the refrigerant from the working solution within the generator. Illustrative examples of air conditioning/refrigeration systems of this general description are set forth in U.S. Pat. Nos. 3,661,200 ("Absorption, Refrigeration and Air Conditioning System", issued to McNamara); 5,231,849 ("Dual-Temperature Vehicular Absorption Refrigeration System", issued to Rosenblatt), and; 5,383,341 ("Refrigeration, Heating and Air Conditioning System for Vehicles", issued to Zur et. al.). However, achieving the necessary heat transfer from the exhaust gases to the absorbent-refrigerant mixture can be problematic. The relatively large surface area required for gas-to-liquid heat exchange, coupled with the need to minimize the exhaust gas pressure drop associated with the heat exchanger (since increasing the back pressure of the combustion gases in the engine adversely affects engine performance) results in heat exchanger or generator designs which are expensive, bulky and difficult to install in vehicles.
An alternative source of waste heat in a vehicular engine is available from the engine coolant circuit. Heat received by the circulating coolant (typically a water-ethylene glycol mixture) is normally dissipated to the environment by the vehicle's radiator. A system utilizing the waste heat from the engine coolant is an attractive alternative to systems utilizing heat from the exhaust gases since no bulky gas-to-liquid heat exchanger is required. U.S. Pat. No. 4,307,575 ("Automotive Absorption Air Conditioner Utilizing Solar and Motor Waste Heat", issued to Popinski) teaches an absorption cycle air conditioner system for use in conjunction with an electric-powered vehicle in which heat transferred from the recirculating engine coolant is utilized to evaporate the refrigerant within the generator. However, this system requires additional heat input from a solar collector mounted on the roof of the vehicle to operate. To the best of the applicant's knowledge, the prior art does not include any examples of vehicular absorption air conditioners in which waste heat from the engine coolant is used to thermally drive the generation step without any additional heat input from other sources.